1. Field of the Invention
The present invention relates generally to the manufacture of projectiles, such as shot, bullets, pellets, and the like, and in particular to a tungsten and iron-based projectile having unique density and softness characteristics, and which can be used in the manufacture of bullets and shot, such as shotgun shot or pellets.
2. Description of Related Art
Presently, projectiles, such as bullets, shot, and pellets, are manufactured from a variety of materials, including many metals, such as lead. However, as the use of lead has decreased, due to well-documented environmental impacts, projectile manufacturers have turned to other metals to replace these lead-based projectiles, such as steel. In particular, various projectiles have been provided, according to the prior art, that are composed of some mixtures of tungsten, nickel, iron, etc. Using these metals, the manufacturer can offer a lead-free and environmentally-safe projectile.
While these prior art lead-free projectiles are useful in many applications, they often have density ranges that are outside the acceptable range for a projectile that effectively emulates a lead bullet or lead shot. Within the small group that yields acceptable density there are no offerings in the current art that are adequately soft and ductile to be used in firearms without special considerations being made. To be more precise, there are no offerings that are adequately soft and ductile to be shotgun-choke responsive. Projectiles made by many of the current manufacturing routes are often much harder than lead and therefore cannot emulate the internal ballistic, external ballistic, and terminal ballistic characteristics of lead-base projectiles and shot.
As one substitute for lead shot pellets, and according to the prior art, steel shot pellets have been developed and are in widespread use. Steel shot falls far short of the density of lead (7.8 g/cc vs. 11.2 g/cc) and therefore has significantly lower performance. Further, these steel shot pellets are significantly harder than lead and therefore are not appropriately deformable and do not typically produce uniform pattern densities, particularly at extended range. Further, special considerations need to be made with regard to the firearm in order for steel shot to be used safely. In order to provide an effective pattern density, shells with variably sized pellets have been produced in order to provide the appropriate pattern density. However, variably sized shot pellets have varying external and terminal ballistics. Accordingly, steel shot pellets are not an effective substitute for lead shot. In all cases with steel shot, performance is significantly limited by the hardness and density of steel.
As is known in the art, in the manufacturing of shot, various powdered metal materials are often compacted and subsequently sintered in order to form the projectile. This prior art can be generally subdivided into several distinct categories.
One category is considered to be frangible, such that the projectiles disintegrate upon impact of the target or backstop and are used mainly for training purposes for law enforcement and military personnel. The disintegration of these projectiles reduces the risk of ricochet and therefore is considered to be a safer choice than other alternatives especially in close range combat simulation. These materials (by design) are brittle and the particles must only be lightly bonded in order to meet the requirements of the application. Some of these materials are relatively porous, however they lack sufficient bonding to impart significant ductility to the resulting projectile. Frangible ammunition utilizing sintering techniques is generally made by one of two methods: (1) low-temperature solid state sintering, in which the temperature remains below the solidus of any of the materials in the mixture; or (2) transient liquid phase sintering, which is a process where bonding occurs as the temperature is elevated above the eutectic of two materials and a temporary liquid is formed. As soon as the liquid forms, it alloys with the other metal and the melting point rises such that there is no longer liquid. The result is light metal-to-metal bonding that relies on the small, weak, and brittle intermetallic compounds that form at the contact points of the particles as a result of passing through the eutectic temperature. Several sintered (non-polymer bonded) variants on these basic methods exist, however, the goal remains the same—brittle bonding to achieve the goal of frangibility.
A second major category of powdered metal approaches to ammunition involves mechanical pressing that serves primarily as a shaping function and sinter-densification to reach the desired density. This second category of approaches utilizes very fine metal particles (some of which may be tungsten and iron) that are sintered at high temperatures (in excess of about 80% of the melting point) or liquid phase sintered in which the sintering temperature is at least above the solidus one of the materials.
In order to densify to near full theoretical density, powders below about 6 microns are generally used. Such methods are commonly employed in the manufacture of tungsten heavy alloy components for a wide range of applications and these methods are well known in the art. This second category of approaches is essentially an adaptation of the technology for production of tungsten heavy alloys for the manufacture of high-density ammunition components and to a large degree employs the same basic techniques and principles, which are well published. Densities greater than lead are possible, with near full theoretical density commonplace, however, these methods produce components with high hardness values that are very similar to or higher than steel.
As is taught by the literature with respect to tungsten heavy alloy production, powdered metals for these approaches are typically very small and spherical or semi-spherical. The small size lowers the necessary sintering temperature and allows near complete densification, however, when powder pressing methods are used, higher levels of polymer are added to compensate for the lack of mechanical interlocking typical for spherical powders. In particular, small semi-spherical powders are not readily compacted in traditional powder metallurgy methods due to a lack of mechanical interlocking during pressing and require relatively large amounts of wax or polymer to adhere the particles. The main reason for this difficulty is that mechanical powder compaction relies largely on deformation and interlocking of large, irregular shaped particles to provide the strength required for ejection from the die. In the case of small semi-spherical powders, the polymer is used as a “binder”, whereas with large irregular powders, it is used at a much lower level as a “lubricant” to assist in ejection and does not impart significant strength to the compacted part.
Typical sintering temperatures for alloys containing tungsten and iron are above 1450° C. and require the use of special high-temperature furnaces. Lower temperatures can be used, however, sintered density is greatly reduced, thus becoming self-defeating. Further, such high-temperature or liquid phase sintering of tungsten alloys requires the use of high levels of hydrogen in the sintering atmosphere in order to reduce the surface oxides present on the powder surfaces. Because the surface area for a given mass increases as particle size decreases and surface oxides are always present at some level, there is a larger proportion of metal oxide present with smaller particles. This oxide must be reduced prior to pore closure during sintering or gases that evolve from the reduction of these oxides will create trapped porosity. This phenomenon is well documented in the literature and is sometimes termed “hydrogen embrittlement” due to the fact that oxides trapped in the interstitial spaces between particles can form water molecules in the presence of hydrogen. These trapped water molecules are too large to escape through the matrix or grain boundaries and therefore increase the brittleness of the material due to pores remaining after sintering. Further, due to the high binder content necessitated by the particle shape, surface oxides are not acted upon by mechanical smearing as much as with larger irregular powders due to the lubricating hydraulic boundary layer effect that the excess binder produces.
In systems with a high and low melting point material, such as tungsten and iron containing systems using high temperature or liquid state sintering processes, significant bonding occurs between the high melting point metals due to the enhanced mobility of the atoms of the high melting point metal within the liquid matrix. However, depending upon several factors, such as solubility limit, the amount of higher melting point metal, processing temperatures, etc., a solid solution may result after cooling, which can have a wide range of microstructural characteristics from fine dispersed grains to very large solid interconnected grains. In the case of a two-metal system in which there is no solubility of the higher melting point metal in the matrix, no solid solution will occur, and sintering relies instead on liquid filling in the spaces between the higher melting point particles. In liquid phase sintering, the liquid that is formed greatly increases the surface contact area between particles and dramatically increases mass transport mechanisms. This subsequently leads to rapid rounding of porosity and densification. The use of smaller particles is beneficial in this type of processing due to the inverse relationship between particle size (diameter) and surface energy, as is well described in the literature. As particle size is decreased, the ratio of surface area to volume is increased, thus creating an energy gradient promoting mass transfer between particles (see FIG. 6). This driving force slows as surface area (and consequently surface energy) is reduced until equilibrium conditions are approached and densification essentially ceases.
Another factor that provides drawbacks to prior art projectiles and shot arises from the sintering temperatures and resulting structures of the mixed compound. For example, many of the mixtures of metals are sintered at a temperature where an alloy, intermetallic, metal matrix, etc. are formed. The need for these higher temperatures and highly reducing atmospheres significantly increases the processing costs associated with this sintering method. The formation of these materials and compounds has particular drawbacks to the resulting softness (or hardness) of the projectile. This type of system, where mass transport is great, can result in the widespread formation of intermetallic compounds in tungsten-iron systems, as tungsten atoms are highly mobile in iron at this temperature range. Higher levels of intermetallic compounds lead to decreasing ductility. In addition to the reduced hardness of the present invention, the larger amount of retained porosity allows for the projectile to be easily deformed by a shotgun choke. This, in turn, improves ballistic performance.